1. Field of the Invention
The present invention relates generally to the preparation of isocyanates and, more specifically, to a low temperature process for the preparation of an isocyanate from an amide using hypochlorous acid.
2. Background of the Invention
Heretofore, the commercial method of choice for the manufacture of isocyanates has been a gas phase conversion by means of the phosgenation of amines. An example of the use of this methodology for the production of isocyanates is provided in U.S. Pat. No. 4,321,402. Unfortunately, phosgene methodology is expensive in view of the cost of the amine raw materials and the risk associated with the use of highly toxic phosgene gas. Non-phosgene routes to the production of isocyanates are highly sought after by the isocyanates manufacturing community.
One alternative to the use of phosgenation is carbonylation as disclosed in the above-mentioned '402 patent. However, this method utilizes high pressure reaction equipment and expensive carbon monoxide as a reactant, and the method typically uses potentially toxic catalysts such as selenium.
Another alternative is the well-known Hofmann rearrangement reaction entailing the base catalyzed rearrangement of N-halo amides to isocyanates. This reaction is typically conducted in single step without isolating the N-halo amide intermediate before it is reacted and converted into an isocyanate. The isocyanate is not Produced and isolated directly but requires the isocyanate to be trapped as a urea and/or a carbamate, which is then in turn converted to the desired isocyanate by hydrolysis and/or pyrolysis.
As yet another alternative, U.S. Pat. No. 4,282,167 discloses the preparation of isocyanates using a modified Hofmann rearrangement reaction by reacting an alkali metal hypobromite or alkali metal hypochlorite with a solution of a substantially water-insoluble aliphatic or cycloaliphatic organic amide in a substantially water-immiscible organic solvent using a quaternary salt as a phase transfer catalyst. Unfortunately, the solubility of such alkali metal hypobromite and alkali metal hypochlorite salts is typically only about 5% by weight in water, and the purity levels of these salts is sometimes less than might be desired.
New, more efficient, non-phosgene processes for producing isocyanates would be highly desired by the isocyanates manufacturing community.